Forgotten Sins

[Poche]

All your  sins are forgiven when you recieve the absolution. Be sorry for all of your sins and Jesus will forgive you.

^ This. As long as you aren't purposefully withholding a mortal sin in Confession, all sins are forgiven after reception of the Sacrament. If you remember a sin you know was mortal afterwards, add it to your next Confession.

Fr. Pablo Straub (God rest his soul) handles this question well:

In the original post he implied that he was not withholding any sins in the confession. He was talking about instances when a sin is forgotten.